1. Field of the Invention
The present invention relates to an information transmission scheme of transmitting a coded video picture/still picture using a cable communication network such as an ISDN (Integrated Services Digital Network) or Internet, or a radio communication network such as a PHS or satellite communication, and a coding/decoding apparatus in an information transmission system adopting this method.
2. Description of the Related Art
Recently, with the advance of a digital coding technology and broadband network technology for various kinds of information such as a picture, applications using these technologies have extensively been developed. A system of transmitting a compression-coded picture and the like using a communication network is being developed.
For example, a videophone, teleconference system, and digital television broadcast adopt a technique of compressing and coding video pictures and speech into small information amounts, multiplexing the compressed video code stream, speech code stream, and another data code stream into one code stream, and transmitting and/or storing the code stream.
As a video signal compression-coding technique, techniques such as motion compensation, discrete cosine transform (DCT), subband coding, pyramid coding, and variable-length coding, and a scheme using a combination of them are developed. The video coding international standard scheme includes ISO MPEG-1 and MPEG-2, and ITU-T H.261, H.262, and H.263. The international standard scheme of multiplexing data and a code stream prepared by compressing video pictures and speech/audio signals includes an ISO MPEG system, and ITU-T H.221 and H.223.
In a conventional video coding scheme such as this video coding international standard scheme, coding is done in units of GOBs (Group Of Block) or macroblocks prepared by dividing a video signal into frames and dividing each frame into smaller regions. Then, pieces of header information representing a coding mode and the like are added to each frame, GOB, and macroblock. These pieces of header information are necessary to decode all the frames, GOBs, and the like.
If errors are mixed in header information in a transmission line/storage medium, and the header information cannot be normally decoded by a video coding apparatus, all the frames, GOBs, and the like including the header information cannot be normally decoded. The quality of a reconstructed video picture in the video decoding apparatus greatly degrades.
More specifically, in transmitting a compression-coded picture using a communication network, the receiving side must execute decoding processing of reconstructing significant information from a transmitted “0”/“1” bit stream.
For this purpose, the above-described header information is very important as information representing the rule of coding a set of predetermined bit steams. Examples of the header information are information representing the prediction type of frame being decoded (whether intraframe coding or interframe coding), time reference information representing the display timing of the frame, and step size information used in performing quantization.
If these pieces of header information are lost, image information transmitted subsequently cannot be normally decoded.
For example, assume that an error is mixed in a bit stream owing to any cause, and the bit pattern changes to represent intraframe coding though the prediction type of frame is supposed to represent interframe coding. In this case, even if subsequent actual information is normally transmitted, the decoding side determines the bit pattern as a result of intraframe coding, and hence cannot normally encode finally, sequentially transmitted information.
Consequently, the quality of a reconstructed video picture in the video coding apparatus greatly degrades.
Mixture of errors frequently occurs in a system, such as a radio videophone, portable information terminal, or radio digital television receiver, that transmits and/or stores a video picture via a radio transmission line.
The mainstream of conventional picture transmission is a system using a cable communication network. Even in the use of a radio communication network, picture transmission assumes satellite communication whose error rate is very low. In light of this, the structure of a coded stream to be transmitted does not sufficiently consider the error resilience, and important information such as header information is not satisfactorily protected against the transmission error.
In a PHS (Portable Handyphone System) expected to become one of the mainstreams of future mobile communication, the error rate is about several hundred thousand to million times that of satellite communication. Therefore, errors cannot be fully corrected only by conventional error protection or correction done for a coded bit stream.
In the Internet expected to become one of the mainstreams of future communication as well as the PHS, time at which an error is mixed and the type of mixed error are not statistically clarified, and no proper error correction may be done.
For this reason, in transmitting a code stream coded using arbitrary shape picture coding, the error resilience of transmission data weakens.
It can be attained by the present invention to make even in an arbitrary shape picture coding to have an error resilience similar to that in a conventional coding method for coding a rectangular picture.